There are classes of applications in petrochemical and power utility industries that involve the thermal imaging and absolute temperature measurement of wall-tube surfaces in direct fired process heaters under production conditions. The temperature range in these processes varies from 400 to 1200° C. It is widely recognized that operation of furnace tubes above their creep-rupture design temperature, for example in ethylene plants, results in diminished lifetimes and increases the prospects for premature failures. Any failure of the tubes results in very expensive repair costs and furnace shut downs.
In addition, temperatures often adversely affect productivity or yield of desired product. For example a 10° C. temperature difference from desired temperature at the coil output of a large ethylene plant can result in hundreds of thousands of dollars of revenue loss per year. Such a loss is caused by a less than desired conversion of the feedstock. The same thing applies to coker furnaces in refineries. When operating at a slightly higher temperature than optimum, overcaking and increased coke formation inside of the tubes results. This causes higher outside temperature, and consequently reduced through puts. Formation of coking inside tubes and identifying the exact location of this formation is most important. Large uncertainties in the tube surface temperatures are unacceptable if the process is to operate under nearly optimized conditions with any degree of confidence.
Power utilities are becoming more cost conscious as the result of deregulation. Much of the same principles detailed above apply to utilities' furnaces. In a coal fired utility furnace, for example, identifying clinker formation inside the boiler tubes is as important as identifying coke formation in petrochemical cracking furnaces.
The industry's techniques for the furnace tube temperature measurement have improved through the years and there exist three methods at the present time for this measurement.
The first method utilizes a thermocouple with direct physical contact, such as welding to the tubes in select locations. However, thermocouple installations are unreliable for extended operation because of the rapid drift in their calibration; and, deterioration of the protective materials in the furnace atmosphere. In addition the number of thermocouples installed is limited due to the complexity which results in the associated wiring and instrumentation. Normally distances of 100 meters and longer are necessary to reach the control room. It is nearly impossible to identify the exact location of tube coking by the thermocouple method.
A second method, which is widely used in many plants employs portable, single point radiation thermometers with appropriate optics, spatial resolution and infrared filtering. These instruments have the ability to correct for the effects of in-furnace conditions such as emissivity, reflected irradiance and furnace gas emission/absorptions on the indicated radiation thermometer readings (see literature for Mikron model M90D and Mikron/Quantum Logic model I, both manufactured by Mikron Infrared, Inc. of Oakland, N.J., (hereinafter “Mikron”, “Assignee” and/or “Applicant”) for more details). In the Mikron/Quantum Logic I a novel method of using a modulated laser permits measuring the emissivity of the tube, allowing more precise temperature measurement of the tube.
These conventional, single point, portable radiation thermometers have one serious shortcoming, i.e., it is nearly impossible to expect someone to measure all the tubes across the entire length or height of the furnace. The number of measurements can easily reach hundreds per furnace per day. Operator fatigue and boredom will eventually result in the deterioration of the quality of the reported data. Consequently the process engineers choose only select points for measurement and ignore the rest of them. Thus, the identification of locations where coke formation takes place, becomes more a matter of chance than a certainty.
The third method presently employed uses a thermal imaging instrument with a sufficient field of view to observe a very large portion of the interior of furnace. FIG. 1 shows a cross section of a typical coker furnace in a refinery. The fields of view 1, 3, 5 for different imager positions 7, 9, 11 are depicted. A sufficient number of viewports 13, 15 are available in order to image a substantial if not all of the interior of the furnace. These mid-wavelength, infrared (MWIR) instruments include a suitable infrared filter which allows the imager to “see” through a substantial depth of hot combustion gases 17. A typical infrared filter is a narrow pass band filter centered at 3.90 um. Flame combustion by-products include gases such as H2O, N2, CO2, and NOx, and a small residue of ashes and other particles. These hot combustion gases emit a substantial amount of radiation toward the wall tubes 19 resulting in heating the tubes. It is known that at 3.90 um there is a void in the spectrum of hot gases radiation (see FIGS. 2A and 2B) that makes the hot gases very transparent. An instrument operating at this particular wavelength where the target is absorptive and thus emissive, can provide a very high quality thermal image of the interior of the furnace even in the presence of hot combustion gases.
In addition by estimating or knowing the emissivity of the tubes and furnace background temperature for calculation of tube reflected irradiance, one can get adequate repeatability. The sensitivity of the thermal imagers is quite good such that differences of 1-2° C. can be easily discerned. However, the matter of establishing absolute temperature levels on tubes is quite another matter.
Modern thermal imagers have the ability to store the images taken in the field for further off-line image processing. A number of useful parameters and in particular temperature profile/time trend analysis can be readily determined. The trend of wall tube temperature in most cases can effectively be used as an indication of the expected life of the tubes or formation of coke inside of the tubes, either one of which having a substantial effect on the productivity of the process and the over all cost of operation of the plant.
Present State of the Art in Thermal Imaging
The existing thermal imagers designed with an appropriate infrared band pass filter of 3.9 um for penetration through hot combustion gases rely on photon detectors such as Indium Antimonite (InSb), Mercury Cadmium Telluride (MCT), Platinum Silicide (PtS) or Quantum Well Infrared Photo-detector (QWIP). A typical detector has an array of 320H×240V elements (pixels) to form a thermal image and are very sensitive in the spectral band of 3 to 5 um. The main shortcoming of this class of detector is that they have to operate at very low cryogenic temperatures, such as 77K, which is equivalent to the temperature of liquid nitrogen.
To achieve cryogenic temperatures for a portable instrument demands a very high-tech cryocooler, which operates on the same principles as a house refrigerator, except that helium gas or other very low temperature liquid gas is used as the medium of compression. In addition to the initial manufacturing costs, incorporating a cryocooler compressor into a portable instrument has other shortcomings. Firstly, in order for a cryocooler to reach sufficiently low cryogenic temperatures it takes several minutes. Second of all, a cryocooler has many moving and sealing parts such as piston, cylinder, gaskets, o-rings and motor. The piston, cylinder, gaskets and o-rings seals must operate under very high pressure, in order to convert gas to liquid. The typical life of a cryocooler is about 2000 hours. A normal failure mode is the leaking of helium gas through the seals. The replacement or repair of a cryocooler can exceed 25% of the initial cost of buying the instrument. Besides being costly, repairs normally are associated with long delays due either to spare parts' shortages or the limited number of repair people with the necessary level of expertise. Further, the battery life is mostly consumed in keeping the detector cooled. Normally operators must carry an external high capacity battery, either strapped over the shoulder, or belted around the waist. This adds to the inconvenience and poses a threat to the safety of the operators, since operators must image the interior of these furnaces from narrow catwalks through hot view ports several stories high. Of course, the avoidance of injuries during this operation is of paramount importance to plant management.
Further, these photon detectors are effectively limited, again, to the spectral band of 3 to 5 um. As such, they are not useful in detecting “lower” temperatures, for example, in the range of −40 to 200° C., and particularly outdoors, during the day, in sunlight, which, due to the influence of the sun, a powerful source of radiation energy at 3 to 5 um, precludes their use. These lower temperatures can occur at other points in petrochemical-related processes and can also be very critical. Monitoring of these conditions is usually accomplished using a long wave infrared (LWIR) imaging radiometer operating in the 8-14 um spectral bands.
Also, ancillary furnace and other facility functions can be the subject of a comprehensive predictive and preventive maintenance (PPM) program requiring a similar low temperature, detection capability.
Thus the present state of the art requires the use of two instruments to cover the broad temperature range of −40 to 2000° C. for two distinctly different applications, such as coker furnaces and PPM activities.
In the last several years a class of un-cooled focal plane array (UFPA) infrared detectors has been introduced to the commercial market for numerous industrial, scientific, security, public safety, automotive and fire fighting applications. The impetus for the design and development of these modern detectors was the need by the military for a light, highly potable night vision device. The main advantage of UFPA detectors is that they can operate at room temperature. There is no need for a cryogenic environment to cool down the detectors.
However all these detectors are optimized to operate for the longer wavelengths of the infrared spectrum, normally beyond 6 um. The detector is vacuum-sealed for optimum performance by an infrared transmitting window covering the sensitive sensing elements (pixels). The infrared transmission characteristic of this window is from 6 to 14 um or 8 to 14 um. The spectral transmission of 8 to 14 um is preferred choice, since the effect of atmospheric absorption band is minimized, thus allowing greater clarity of images at longer distances.
It is a primary object of this invention to provide for a new use for these UFPA devices by adapting them in a way that permits them to be used as MWIR devices in addition, so that they are able to measure the high temperature of target surfaces, for example the tube walls inside a furnace without the interference from combustion flames, as well as functioning in low temperature ranges, including detecting temperatures in broad daylight.
It is a further object of the present invention to provide a portable lightweight instrument which does not require cooling to cryogenic temperatures thereby prolonging instrument battery life thereby providing additional convenience and safety that is very desirable in such environments.
A still further object is to adapt existing UFPA thermal imaging devices so as to accomplish the purposes of the present invention, the device including built-in firmware and associated off-line software, for example, MikroSpec™ off-line software, to further enhance the degree of accuracy of the measurement, allowing for further temperature/time trend analysis which can for example prolong the life of the tubes and thus the productivity of plant operation, or provide other benefits when used with different processes.